Light valves have been known for over sixty years for use in the modulation of light. Light valves have been proposed for use in numerous applications during that time including, e.g., alphanumeric displays, television displays, windows, sunroofs, sunvisors, mirrors, eyeglasses and the like to control the amount of light passing therethrough. Light valves of the type described herein are also known as “suspended particle devices” or “SPDs”.
As used herein, the term “light valve” is used to describe a cell formed of two walls that are spaced apart by a small distance, with at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent conductive coatings. The cell contains a light-modulating element which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distributed and encapsulated.
The liquid suspension (sometimes herein referred to as “a liquid light valve suspension”) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension exhibit random Brownian movement. Hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state.
For many applications, it is preferable for the activatable material, i.e., the light-modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film, in which droplets of liquid suspension are distributed, is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging associated with a high column of liquid suspension, can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film, the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A “light valve film”, as that term is used herein, refers to a film having droplets of a liquid suspension of particles distributed in the film.
U.S. Pat. No. 5,409,734 exemplifies a type of light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking emulsions are also known. See U.S. Pat. Nos. 5,463,491 and 5,463,492, both of which are assigned to the assignee of the present invention.
For use in set suspensions such as light-polarizing sheets, sometimes called “sheet polarizers”, which can be cut up and formed into polarized sunglass lenses or used as filters, light-polarizing particles can be dispersed or distributed throughout a sheet of suitable film-forming material, such as cellulose acetate, polyvinyl alcohol or the like. Methods of making set suspensions for use in sheet polarizers are well known in the prior art. It is important to note, however, that the light polarizing particles mentioned above are immovable, i.e., fixed, in such set suspensions. See, e.g., U.S. Pat. Nos. 2,178,996 and 2,041,138.
To facilitate a better understanding of suspended particle devices, such as those produced in accordance with the present invention, a description of the liquid light valve suspension is provided below, with details concerning the components thereof, including the liquid suspending media, the stabilizers optionally included therein and the particles suspended in the media.
A liquid light valve suspension for use in the invention may be any liquid light valve suspension known in the art and may be formulated according to techniques known to one skilled in the art. The term “liquid light valve suspension” as used herein means a “liquid suspending medium” in which a plurality of small particles is dispersed. The “liquid suspending medium” comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
The liquid light valve suspension useful in the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful herein, include but are not limited to the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175 and 4,407,565. In general one or both of the liquid suspending medium or the polymeric stabilizer dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles but which also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, there may be two or more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles, together with one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light-absorbing or light-reflecting.
Conventional SPD light valves have generally employed polyhalide particles of colloidal size. That is, the particles generally have a largest dimension averaging about 1 micron or less. Preferably, most polyhalide particles have their largest dimension less than one-half of the wavelength of blue light i.e., 2000 Angstroms or less to keep light scatter extremely low.
A detailed review of prior art polyhalide particles can be found in “The Optical Properties and Structure of Polyiodides” by D. A. Godina and G. P. Faerman published in The Journal of General Chemistry, U.S.S.R. Vol. 20, pp. 1005–1016, (1950).
Herapathite, for example, is defined as a quinine bisulfate polyiodide, and its formula is given under the heading “quinine iodosulfate” as 4C20H24N2O2.3H2SO4.2HI.I4.6H2O. in The Merck Index, 10th Ed. (Merck & Co., Inc., Rahway, N.J.). In polyiodide compounds, the iodide anion is thought to form chains and the compounds are strong light polarizers. See U.S. Pat. No. 4,877,313 and Teitelbaum et al. JACS 100 (1978), pp. 3215–3217. The term “polyhalide” is used herein to mean a compound such as a polyiodide, but wherein at least some of the iodide anion may be replaced by another halide anion. More recently, improved polyhalide particles for use in light valves have been proposed in U.S. Pat. Nos. 4,877,313, 5,002,701, 5,093,041 and 5,516,463. These “polyhalide particles” are formed by reacting organic compounds, usually containing nitrogen, with elemental iodine and a hydrohalide acid or an ammonium or alkali metal halide or alkaline earth metal halide. Such organic compounds are alternatively referred to as “precursors.”
However, in order to be commercially useful, it is necessary that the particles used in a liquid light valve suspension, whether or not incorporated into a film, have great chemical and environmental stability. To obtain particles of great environmental stability, in turn, it may be desirable to use non-polyhalide particles in light valve suspensions and films, especially where the stability of the material composing the particles is known to be excellent.
The use of graphite particles in light valves was first proposed in U.S. Pat. No. 1,963,496. However, an attending disadvantage of employing conventional graphite particles in light valve suspensions or films is that graphite particles are prone to rapidly agglomerate when a light valve suspension of them is activated. In addition, prior art graphite particles are also known to scatter an unacceptably large amount of light as well as settling due to the gravitational pull.
Many kinds of known pigments are used as colorants in paints, varnishes and inks. Generally, such pigments have average particle sizes in the range of 0.1 to 10 microns, but usually 1 micron or greater. To obtain these small sized particles, mechanical devices are often used to comminute solid particles into smaller ones. Ball mills, attritors, and bead mills and roll mills are commonly used for such purposes.
The use in a light valve suspension of particles produced by such mechanical methods is not practical, however, for several important reasons. First, the particles are usually too large, generally having an average size (diameter) of 1 micron or greater. Secondly, even if sub-micron sized particles are produced, the grinding and comminution process tends to make such particles spherical in shape or amorphous, reducing or virtually eliminating their aspect ratio. For use in a light valve suspension anisometrically shaped particles are important; hence, particles shaped like needles, rods, flakes or plates and the like are preferred and generally necessary because their anisometric shape facilitates orientation in an electric or magnetic field. Thirdly, an inherent disadvantage of mechanical comminution is that there is a large distribution of sizes, generally resulting in the presence of particles having a size of 1 micron or more even if the average particle size is less than 1 micron. Particles larger than 0.2 micron (one-half the wavelength of blue light) tend to scatter light, and such scattering increases exponentially with particle size. This fact and the fact that relatively large particles promote agglomeration, are additional reasons why such comminuted particles are not desirable for use in a light valve suspension. There is thus a need in the art for light valves containing a light valve suspension of ultrafine particles.
Moreover, while various types of particles have been suggested in the prior art for use in light valves, heretofore it has not been practical to obtain particles of a submicron size and anisometric shape and good optical properties, except for polyiodide particles. However, in some cases polyiodide particles may not be sufficiently stable to ultraviolet radiation, and light valve suspensions of such polyiodide particles may degrade in terms of color and performance if exposed to intense ultraviolet radiation for a prolonged period of time unless special measures are taken to protect against UV radiation. Such measures may include, for example, UV absorbers in the light valve suspension or film or using a UV filter to intercept UV radiation before it strikes the suspended particle device. Also, nearly all polyiodide particles are limited to a blue color, whereas it is also desirable to have light valve suspensions which have off-state colors which are not blue. Accordingly, new types of particles for light valve suspensions are needed which will be suitably small and anisometrically shaped, which will tolerate high levels of ultraviolet radiation for long periods of time without significant degradation and/or which have a variety of off-state colors.
U.S. Pat. No. 5,650,872 provides an electro-optical device, such as a light valve or electrophoretic display, comprising a cell formed of opposed cell walls, a light-modulating unit comprising a suspension containing anisometric particles prepared by the Evaporative Dispersion Process suspended in a liquid suspending medium between the cell walls, and opposed electrode means operatively associated with the cell walls for applying an electrical field across the suspension. The anisometric particles have an average particle size of about 0.2 microns or less, preferably about 0.1 micron or less.
The liquid light-modulating suspensions used in U.S. Pat. No. 5,650,872 have a wide array of off-state colors and can include materials superior to polyiodide particles in terms of their ability to tolerate high levels of ultraviolet radiation for long periods of time without serious degradation. However, all of the particles made in accordance with U.S. Pat. No. 5,650,872 must be made with the Evaporative Dispersion Process, and this process is not suitable to form sufficiently small anisometrically shaped carbon or graphite particles because the process, which is disclosed and described in U.S. Pat. Nos. 5,030,669 and 5,106,533 only is useful for non-elemental particles, i.e., defined in the cited patents and used herein to refer to particles comprising more than one element.
The present invention attempts to overcome the disadvantages of using conventional carbon and/or graphite particles in light valve suspensions, the achievement of which becomes apparent from the discussion appearing below.